1. Technical Field of the Invention
The present invention relates to a porous sound-absorbing material used for the exhaust nozzle and the like of a jet engine, in particular to a porous sound-absorbing material with a high sound-absorbing effect over a wide frequency range and its manufacturing method.
2. Description of the Related Art
Conventional porous sound-absorbing materials used in the exhaust nozzle and the like of a jet engine are composed of a honeycomb structure as shown in FIG. 1 Partial isometric view of a sound-absorbing material for high-temperature uses.
The honeycomb sound-absorbing material shown in FIG. 1 is comprised of a honeycomb 1 made of a heat-resistant alloy, a perforated plate 2, and a rear plate 3; the material has a reactive structure like a Helmholtz resonator, and disperses sound energy using the friction on the wall surface and the kinematic energy produced when the media move.
With the honeycomb sound-absorbing material described above, there have been problems such as the perforated plate 2, inner honeycomb 1 and its rear plate 3 being overheated or greatly deformed by heat from high-temperature exhaust gas (for instance, 700˜800° K or more) For example, since the perforated plate 2 is made of a stainless steel panel or an aluminum panel, the plate may be damaged or thermally deformed, and the brazed portion between the plate and honeycomb 1 may peel.
Furthermore, a sound-absorbing material with the structure described above provides only a narrow frequency range of absorbable noise, so such a wide range of noise as produced by a jet engine (for example, 1,000˜3,000 Hz) cannot be absorbed satisfactorily.
In these circumstances, foamed sound-absorbing materials that can resist heat and provide a wide sound-absorbing frequency range have been proposed. These foamed sound-absorbing materials are of a resistance type made of porous ceramics, as shown in the enlarged photo in FIG. 2 (for instance, unexamined Japanese patent publications No. 187163, 1998 and 194864, 1998).
According to the above-mentioned conventional “Light-weight ceramic sound-absorbing material and its manufacturing method” of the unexamined Japanese patent publication No. 187163, 1998, both resistance to heat shocks and sound-absorbing performance of a sound-absorbing material are made high by laminating a dense layer containing ceramic fibers on the surface of a porous foamed ceramic. However, because this material is composed of multiple layers of several types of sound-absorbing material, there is a problem in that its manufacturing cost is high. Moreover, since the sound-absorbing material is manufactured by foaming a ceramic, its porosity is not constant; therefore, as shown in the frequency characteristic graphs in FIGS. 3A and 3B, there is also another problem in that the sound absorption effect cannot be easily expanded to a wide range of sound frequencies.
“Light-weight CMC sound-absorbing material and its manufacturing method” of the unexamined Japanese patent publication No. 194864, 1998 discloses a porous material made of an alumina-based ceramic containing SiC whiskers with a porosity of 80˜92%, in which acoustic absorptivity is increased by adding silicon carbide fibers without changing the specific bulk density and controlling flow resistance. However, as this sound absorbing material is also manufactured by foaming a ceramic, porosity is not constant, so one problem is that a satisfactory sound-absorbing effect cannot be obtained over a wide sound frequency range.
The present invention aims to solve these problems. The objects of the present invention are to provide a porous sound-absorbing material that is light and highly resistant to heat for application to the exhaust nozzle and the like of a jet engine with a high sound absorbing effect to noises over a wide range of sound frequencies, such as those of a jet engine, and also to provide a method of manufacturing such a porous sound-absorbing material as that described above with stable performance and porosity without variations.